"We require Mac OS X because the products we make run on OS X and we believe in testing on the same hardware our customers use, this helps produce a better product".
If the software they're testing is used mainly on Macbook Pro's, then they're wanting to test on Macbook Pro's. Things like GPU and CPU differences may make a difference to how their software performs.
"We have data centers with thousands of machines configured with all 3 OS’s running constant build and test operations 24 hours a day 365 days a year. This is just a small look at the Mac side of things."
Any Mac hardware will do like Mini's, xServes, and Mac Pros. Mac Pro's work really well and if you just rent them from a data center, the capital expenditure isn't there so you don't get hit with the $3k price tag.
Without knowing what product is being testing, how can you say that "Any Mac hardware will do"? Like I said - if they're wanting to test on the specific hardware that their customers are running the software on, you can't know that for sure. A Mac Mini has a different CPU/GPU to an XServe, likewise a Mac Pro, likewise a Macbook Pro.
Sorry to offend; I was referencing the "We require Mac OS X" part more liberally. It looks like this author could use any hardware as long as they had HDMI enablers to enable the GPU's in headless servers like the Xserve and Mac Pro.
I don't think they're opening up each row of MacBook Pro's continuously for they're software though. I think they've just got the lids open to make sure the GPU is enabled for better performance in remote sessions.
There are ways around the lid opening thing as far as activating the GPU (same issue applies to Minis and Pros that have no monitor attached in a datacenter environment).
The big requirement for keeping the lid open is probably to control the temperature. You can run a MBP with the lid closed, but it will get pretty toasty, and these aren't being used for a typical laptop workload one presumes.
Build and test is cool. Doesn't need screens, though. Unless you're actually testing the graphics card, in which case you might as well use the Mac Pro.
Potential reason is that they need see how their tests affect screen burn. They mention they specifically need Mac hardware so this could be why they specifically need retina screens.
It might have more to do with running GPU-assisted tasks through remote sessions. You need to either leave a screen attached and on or trick the device into thinking one is attached and on to take full advantage of the GPU on Mac hardware. You can actually do it with any mac hardware (even headless devices like Mac Mini's and Mac Pro's) but it's not advertised well enough.
For example, with mac Mini's in a server environment, you need to plug in a hdmi enabler to get full GPU performance front the system while using it over remote access.
The HDMI dongle is a good solution for most users. It can also be worked around in software (although it is not trivial). It's kind of annoying that OS X requires jumping through these hoops, but at least there's a few solutions out there.
If the software they're testing is used mainly on Macbook Pro's, then they're wanting to test on Macbook Pro's. Things like GPU and CPU differences may make a difference to how their software performs.